Microelectronic devices are typically fabricated, in part, by forming features on selected layers of a semiconductor wafer. The individual features are often formed by patterning a mask to protect selected portions of an underlying layer of material during subsequent processing steps. Various methods of the present invention can be used to fabricate virtually any type of microelectronic device, but such methods are particularly useful for fabricating field emission displays ("FEDs") in use, or proposed for use in computers, television sets, camcorder view finders, and a variety of other applications.
FEDs are one type of flat panel display in which a baseplate with a generally planar emitter substrate is juxtaposed to a face plate with a substantially transparent display screen. The baseplate has a number of emitters formed on the emitter substrate that project from the emitter substrate towards the face plate. The emitters are typically configured into discrete emitter sets in which the bases of the emitters in each emitter set are commonly connected. The baseplate also has an insulator layer formed on the emitter substrate and an extraction grid formed on the insulator layer. A number of holes are formed through the insulator layer and extraction grid, in alignment with the emitters to open the emitters to the face plate. In operation, a voltage differential is established between the extraction grid and the emitters to extract electrons from the emitters.
The display screen of the face plate is typically coated with a substantially transparent conductive material to form an anode, and the anode is coated with a cathodoluminescent layer. The anode draws the electrons extracted from the emitters through the extraction grid and across a vacuum gap between the extraction grid and the cathodoluminescent layer of material. As electrons strike the cathodoluminescent layer, light emits from the impact site and travels through the anode and the glass panel of the display screen. The emitted light from each of the areas becomes all or part of a picture element. Exemplary structures and methods of forming such structures are described in U.S. Pat. Nos. 5,676,853 and 5,695,658, the disclosures of which are incorporated by reference herein.
One objective of FEDs is to produce a desired brightness of light in response to the emitted electrons. The brightness at each picture element depends, in part, upon the density of emitters in the emitter sets corresponding to each picture element. In general, it is desirable to have a constant emitter density from one emitter set to another, and also from one area in an individual emitter set to another. Thus, it is desirable to space the emitters apart from one another by a substantially uniform distance and to make the emitters substantially the same size and shape.
One method for forming emitters is to randomly distribute a number of beads on a hard oxide layer that has been deposited over the emitter substrate. The beads may be distributed across the surface of the oxide layer by depositing a solution in which the beads are suspended onto the oxide layer, and spinning the substrate to spread the solution thereover. Subsequently, liquid is evaporated or removed from the solution to leave the beads on the oxide layer. This generally leaves the beads with a density that goes inversely proportional to the square of the radial distance from the spinning axis. Accordingly, spin coating does not give a uniform density. The beads may also be distributed across the surface of the oxide layer by a dry dispensing method in which a dry mixture of beads is propelled toward the oxide layer in a jet of air or inert gas. The mixture is then allowed to settle on the oxide layer to form a mask of randomly-distributed particles on the surface of the oxide. The oxide layer is then selectively etched relative to the mask to form a random distribution of island-like oxide areas under the beads. After the beads are removed from the oxide layer, the emitters are formed under the island-like areas of oxide by etching, in some cases isotropically, the substrate.
Problems in the past with the above-described methods relate to variations in emitter sizes and shapes. Specifically, the desired diameter of the base of each emitter is generally the diameter of a single, isolated bead. However, in various application methods, the beads often agglomerate into clusters that remain intact as they are distributed across the surface of the substrate. The agglomerated clusters can include beads that are clumped together in a plane over the substrate and/or beads that are stacked into more than one level over the substrate. It will be appreciated that clusters of beads produce larger, irregular-shaped islands of oxide which result in larger, irregular-shaped emitters. As a result, the emitters produced by this emitter patterning method may not have a uniform size and shape.
Another problem associated with various emitter patterning techniques is that the emitters may not be uniformly spaced apart from one another. Specifically, since the beads are distributed randomly across the surface of the oxide layer, it is difficult to control the spacing between the beads. Thus, the space between the emitters produced by this emitter patterning method can vary significantly from one area to another on the display.
Accordingly, this invention arose out of concerns associated with providing improved methods of forming mask patterns. Specifically, this invention arose out of concerns associated with providing improved methods of forming field emitter tip masks.